As a wind turbine blade sweeps around the ‘rotor disc’, it experiences changes in wind speed and direction as a result of wind shear, tower shadow, yaw misalignment and turbulence. As rotor sizes increases with respect to the typical sizes of turbulent eddies, the importance of turbulent wind speed variations across the rotor disc becomes greater.
These variations result in a large once-per-revolution, or 1P, components in the blade loads, together with harmonics of this frequency, i.e. 2P, 3P, 4P and so on. With a three-bladed rotor, these load components will be 120° out of phase between the three blades, with the result that the hub and the rest of the structure will experience the harmonics at 3P, 6P, etc., but the 1P components and the other harmonics will tend to cancel out.
However, this cancellation relies on assumptions of stationary and linearity, but as wind turbines become larger with respect to the length scales of the turbulence, these assumptions become less valid.
This means that the asymmetric loads resulting from 1P components and other harmonics no longer cancel out, and load components at these frequencies can contribute very significantly to fatigue loads on the hub, shafts, yaw bearing, tower, etc.
For reducing said damaging effects, the prior art teaches the use of an individual pitch control added to the collective pitch control and also yaw control. The pitch and/or yaw commands for reducing said asymmetric loads are calculated using measurements of said loads or of the displacements caused by them.
The 1P load components are particularly significant on large wind turbines and, in principle, it should be possible to reduce these by means of individual blade pitch action at the 1P frequency, 120° out of phase at the three blades. This individual pitch action at 1P frequency may be calculated by a control algorithm that uses, as input, the blade out-of-plane loads.
An example of this technique can be found in US 2006/0002792, which is incorporated by reference, where a method to reduce the loads and to provide yaw alignment in a wind turbine that includes the measurement of displacements or moments of the asymmetric loads in a wind turbine is described. Those measured moments or displacements are used to determinate the pitch angle contribution that will try to reduce or counteract the asymmetric rotor loads and to an easy alignment of the yaw system.
If the measuring devices of said displacements or moments fail, and consequently said individual pitch action cannot be implemented, the prior art teaches the operation of the wind turbine at a lower production level for reducing the asymmetric rotor loading until the failure is repaired.
In the case of off-shore wind turbines, the repair of the measuring devices may be delayed long periods of time involving important production losses.